1. Field of the Invention
The invention relates to a backlight module, an over-temperature protection circuit, and an over-temperature protection method thereof. More particularly, the invention relates to a backlight module in which a light-emitting diode (LED) serves as a backlight source, an over-temperature protection circuit, and an over-temperature protection method thereof.
2. Description of Related Art
With the advancement in semiconductor industry, flat panel displays (FPDs) gradually replace conventional cathode ray tube (CRT) displays and have become the mainstream of displays in the market. Liquid crystal displays (LCDs) are the most mature and popular displays among the FPDs. Nonetheless, LCD panels are not capable of emitting light, and therefore backlight modules need to be configured below the LCD panels for providing light sources and achieving display functions. Due to the advantages of long life span, high efficiency, and low environmental pollution, LEDs often serve as the backlight sources of the backlight modules in the LCDs.
In general, a driving circuit of a LED generates a driving voltage via a DC-DC converter, so as to drive the LED to emit light. The driven LED simultaneously emits light and generates heat. Accordingly, the temperature of the LED is raised, and a forward voltage and a forward current of the LED are lowered down together with the increase in the temperature of the LED. Since the driving circuit is not able to sense the temperature of the LED, the driving circuit of the LED cannot make proper adjustment in response to changes to the temperature of the LED. Thereby, when the temperature of the LED is raised, the driving voltage and the current flowing through the LED stay unchanged. Consequently, the excessively high temperature of the LED easily leads to deterioration of the electrical property and reduction of the life span of the LED. To resolve said issue, an over-temperature protection circuit has been installed in the backlight module according to the related art.
FIG. 1 is a schematic view illustrating a system of a conventional backlight module. With reference to FIG. 1, in a backlight module 100, a DC-DC converter 110 generates a driving voltage VD to drive LEDs Da˜Dc. Here, a current ID flows through the LEDs Da˜Dc. Transistors TR1 and TR2 and a thermal resistor RPTC together constitute an over-temperature protection circuit. Here, the thermal resistor RPTC measures the temperature of the LEDs Da˜Dc and has a positive temperature coefficient. When the temperature of the LEDs Da˜Dc falls within an acceptable range, a voltage Va is changed in response to variations in the resistance of the thermal resistor RPTC, and thereby the current ID can be adjusted. However, in order not to turn on the transistor TR2, the voltage Va is set to be less than a forward bias VBE at a base and an emitter of the transistor TR2. For instance, the voltage Va is set to be 0.5 V˜0.69 V.
When the temperature of the LEDs Da˜Dc goes beyond the acceptable range, the excessive resistance of the thermal resistor RPTC results in the fact that the voltage Va is greater than or substantially equal to the forward bias VBE of the transistor TR2, such that the transistor TR2 is turned on. At this time, a voltage Vb at a base of the transistor TR1 is coupled to a ground voltage, and a pulse width modulation (PWM) signal SPWM controlling the transistor TR1 is equivalent to be grounded because the transistor TR2 is turned on. As such, the transistor TR1 is turned off, the current ID cannot flow through the LEDs Da˜Dc, and the LEDs Da˜Dc cannot emit light.
When the temperature of the LEDs Da˜Dc again falls within the acceptable range, the resistance of the thermal resistor RPTC returns to an appropriate value, so as to allow the LEDs Da˜Dc to emit light normally. Although the over-temperature protection circuit can perform the function of protection, the power consumption of the thermal resistor RPTC poses an impact on the conversion efficiency of the DC-DC converter 110 due to the structure of the over-temperature protection circuit. Moreover, the over-temperature protection circuit protects the LEDs Da˜Dc by turning off the transistor TR1. In this case, the LEDs Da˜Dc are likely to be switched to emit light or not, which may cause flickers and unfavorable visual effects.